Earthquakes

An earthquake is a release of energy within the earth’s crust, creating seismic waves that can
cause significant damage. Earthquakes are triggered when unexpected movement along a fault occurs,
volcanic activity increases, or along boundaries of tectonic plates. Although ground shaking can
occur by many processes, such as volcanic eruptions, the majority of earthquakes are related to
movement along faults.

The largest earthquake recorded to date in Wyoming occurred Aug. 18, 1959, in Yellowstone National Park.
The earthquake registered as a magnitude 6.5 and is considered to be an aftershock of the magnitude 7.5 Hebgen
Lake earthquake in southwestern Montana.

Quaternary Faults

A fault is a shear fracture or a zone of fractures in the earth’s crust that
show evidence of displacement along blocks of crust. Sudden movement on a
fault will release energy causing an earthquake.

Quaternary faults are recognized on the surface that have evidence of
movement in the last 1.6 million years and are considered to have the
greatest potential to be the sources of future earthquakes.
Quaternary faults that show movement during the past 10,000 years
are considered active. Approximately 80 Quaternary faults are mapped
in Wyoming, with 26 considered active.

Induced Seismicity

Earthquake activity can be triggered by a number of sources. Volcanic activity, landslides,
and movement along a fault are examples of natural causes of earthquakes. In rare instances,
anthropogenic (human) influences can also cause an earthquake; these events are referred to as
“induced seismicity.” Induced seismicity can potentially be triggered by a variety of industry
processes used in the extraction of energy resources, including mine subsidence and blasts, oil
and gas field depletions, fluid injection for secondary oil recovery, wastewater disposal,
groundwater extraction, and reservoir impoundment.

WSGS hazards geologists completed a study on the potential link between injection/disposal
well activities by industry and earthquakes in Wyoming. Computer-based subsurface modeling
software allowed geologists to map wells operating between 1984 and 2013 along with recorded
earthquake events during the same time period to identify potential relationships. The report
can be downloaded here: